1. Field of the Invention
The present invention relates to a process for preparing polymers useful for powder coatings and is especially concerned with a thermosetting hydroxyl polyester resin which has more than 2 hydroxyl groups at the terminal of its polymer chain and is prepared using blocked isocyanate as a curing agent. Also, the present invention relates to powder coating compositions containing the above polymers.
2. Description of the Prior Art
The polyester resin among various powder coatings has a glass transition temperature of at least 50.degree. C. which allows for storage stability and when being cured, emphasizes the characteristics of flow as a coating media to provide outstanding appearance and gloss at its surface. Therefore, the appearance of coating surface is improved and orange peel phenomena is minimized because the melt viscosity of resin is lower.
In the conventional polyester resin as used in powder coating to date, the glass transition temperature (Tg) is increased above 50.degree. C. and melt viscosity is also increased. Therefore, the coatings are not flowed-out well during curing and appear to have orange peel phenomena and occasionally blister and pin hole phenomena.
There are two methods to finish steel with coatings; pre-coating and post-coating. The steel coated with pre-coating method is called PCM and the paints for use in this method are PCM coatings. When using powder coatings for PCM, it can save time to paint, since the product is derived directly from machining the pre-coated steel. This avoids tediousness, maximizes the productivity resulting from mass production systems.
The conventional hydroxyl polyester resins are produced by using polycarboxylic acid such as terephthalic acid and isophthalic acid, and polyhydric alcohol such as neopentyl glycol, trimethylol propane, propylene glycol and cyclohexane dimethanol. However, these kinds of resins have relatively higher melt viscosities usually of 6000 cps at 160.degree. C. and occasionally more than 10000 cps. The high melt viscosity restricts the flow of molten coating and moreover degrades gloss and appearance. Also, the speed of cure is relatively slower than that of crystalline polyester at certain temperature so that it is inclined to have poor physical properties compared with crystalline polyester. Therefore, higher cure temperatures or more cure time is required to obtain better physical properties.
In the conventional crystalline resin, crystallinity is so emphasized that the coating appearance at the surface is relatively better, but the chemical resistance and resistance against weather, solvent, stain, and salt spray is adversely affected. Therefore, it is known to be very difficult to make high quality coatings with good appearance. For example, European Patent No. 0070118 describes producing crystalline polyester resin using terephthalic acid, isophthalic acid, and 1,6-hexanediol. In this case, better appearance and gloss with good storage stability compared with the conventional resin is provided since the resin has an average molecular weight of 700 to 3000, a melt viscosity at 160.degree. C. of 50 to 3000 cps, and a hydroxyl number of 30 to 160. However, resistance against weather, solvent salt spray and chemical resistance are still poor. Moreover, stain resistance and bending characteristic absolutely required to upgrade quality are so poor that it is somewhat difficult to use the polyester as a high quality PCM (Pre-Coated Metal) coating.